Generally, an engine for an automobile or the like is designed such that fuel consumption rate is best in an operation with load percentage of about 70-80% and at revolution speed of about 40-60% to maximum. It turns out therefore that, in a normal running, the engine is operated at an inferior fuel consumption rate with load lower than design value.
Conventionally proposed countermeasure thereto is an operation with reduced number of cylinders where the number of cylinders ignited in an engine is reduced depending upon output required; the engine is driven only with the reduced number of cylinders and with enhanced load condition so as to improve the fuel consumption rate and the like.
As a countermeasure to a nowadays problem that exhaust purifying catalyst incorporated in an exhaust pipe hardly attains its active temperature during low load running, an operation with reduced number of cylinders during the low load condition has been proposed so as to keep the exhaust temperature at higher level.
This kind of operation with reduced number of cylinders has been disclosed, for example, in the following References 1 and 2.                [Reference 1] JP 2004-137932A        [Reference 2] JP 2004-360577A        